1. Field
This invention relates generally to pH reference electrodes and specifically to such electrodes which have a liquid electrolytic salt bridge which provides electrical continuity between an internal reference electrode and a fluid, the pH of which is to be determined.
2. Prior Art
The hydrogen ion concentration of a fluid is commonly determined with a pH electrode and a pH reference electrode. Each electrode has a lead in electrical contact with appropriate terminals of an electrometer and, in use, both electrodes are brought into electrical contact with a fluid, the pH of which is to be determined, and an electrical circuit is completed. The pH reference electrode provides a stable and accurate voltage against which the output of the pH electrode, as varied by hydrogen ion activity, may be measured. The construction of the pH electrode is such that it "senses" hydrogen ion activity in the fluid and, ultimately, relates that activity to hydrogen ion concentration which is readily expressable in pH units by the electrometer.
Various configurations for pH reference electrodes are well known. The essential features of such electrodes are an internal reference electrode having a lead connectable to an electrometer and an electrical "bridge" which provides electrical continuity between the internal reference electrode and a fluid, the pH of which is to be measured. A very common bridge consists of a salt solution, often saturated. Such a solution can be found in so-called saturated Calomel Reference Electrodes (S.C.E.). Operability of a pH reference electrode having a liquid electrolytic salt bridge requires that the salt bridge solution be readily accessable to the fluid with which the pH reference electrode is contacted. This is readily accomplished by containing the electrolyte in an electrode housing which has a small orifice through which electrical continuity is established. See, for example, U.S. Pat. No. 3,505,196. Such an orifice is often covered with a semi-permeable membrane (e.g. cellophane) which helps keep materials such as proteins from plugging the orifice.
The use of a "saturated" solution as an electrolytic salt bridge assures a constant concentration of the salt bridge and, hence, reliable pH determinations, especially for precious fluids such as blood for which precise pH determinations are often required. In using electrodes having a saturated salt solution as the electrolytic salt bridge, it is a common practice to include crystals of the salt within the saturated solution to assure prolonged saturation with repeated use of the electrode. Unfortunately, however, the use of a saturated salt bridge solution, either with or without added salt crystals, often results in the formation of additional salt crystals within the solution which can block the orifice through which electrical continuity is maintained. Because of such undesirable crystal formation, it is sometimes necessary, prior to using the electrode, to immerse it in warm water to dissolve crystals which may be blocking the orifice. Although the above step has been commonly accepted as a necessary inconvenience associated with using saturated salt bridge solutions, there have been developed various techniques and pH reference elctrodes which tend to avoid the problems associated with using saturated solutions.
For example, in Canadian Pat. No. 878,722, there is disclosed a reference electrode which does away with the need for a saturated solution by incorporating KCl crystals within a polymeric matrix which is in electrical contact with an internal reference electrode. Further, in an article by A. J. Maas, Clinica Chemica Acta, 28, 373-390 (1970) it has been suggested that reliable pH measurements of body fluids (e.g. blood) can be made by using an isotonic NaCl solution as the salt bridge (e.g. 0.16 moles/liter NaCl). See also the description of the preferred pH reference electrode in U.S. Pat. No. 3,763,422, incorporated herein by reference. Another method of avoiding problems associated with the formation of undesired crystals within a pH reference electrode involves immersing a "flow-through" pH electrode (e.g. U.S. Pat. No. 3,357,910) containing a blood sample and a pH reference electrode in an open beaker containing a saturated KCl solution and then reading the measured pH of the sample within the flow-through pH electrode. For examples of such a system see a catalogue (SI-20) entitled, "Scientific Instruments", dated March, 1973 and published by Corning Glass Works, Scientific Instruments, Medfield, Massachusetts 02052.
In using an open beaker containing a saturated KCl solution, a constant salt bridge concentration, and, hence, a reliable pH measurement is assured. However, it has been recognized that the use of such an open beaker results in so-called "KCl creep" which involves the growth of KCl crystals up the sides of the beaker and this results in an unsightly mess which should be cleaned frequently to avoid damage to laboratory equipment and/or instruments. A novel method for avoiding KCl creep is disclosed in U.S. patent application Ser. No. 408,301 filed on Oct. 23, 1973, entitled "Electrolyte Container for pH Measurements", assigned to the present assignee, and incorporated herein by reference. In the above-entitled patent application, it is disclosed that KCl creep can be avoided by maintaining the saturated KCl solution in an essentially air-tight container into which both a flow-through pH electrode and a pH reference electrode can be inserted.
Although the above disclosures suggest various ways for assuring reliable pH measurements when various salt bridges are used, it is significant that the above methods of electrodes, in one way or another, avoid the problems associated with saturated electrolytic salt solutions by not using a liquid bridge (Canadian Pat. No. 1,274,349), using separate containers for the saturated solution (e.g. U.S. Ser. No. 408,301), or requiring the use of isotonic salt solutions (e.g. A. J. Maas, above on the preferred pH reference electrode of U.S. Pat. No. 3,763,422). Quite surprisingly, we have now found that the problems associated with saturated solutions need no longer be avoided by the rather elaborate techniques already devised. Rather than provide another elaborate method for avoiding the problem, we have surprisingly solved it with the novel pH reference electrode described in detail below.